I am currently reading Lyle's book "The Ketogenic Diet" and something bugs me: Why do we need protein on this diet? (The goal is fat loss only on a SKD * )

He states that the brain/body needs 100grams of glucose per day for energy while not in ketosis.

He also states that from the first few days of very low carb to the 3 week mark, the brain uses energy from 100% glucose (100 grams, 0 from ketones) down to 25% glucose (40 grams, rest from ketones).

He says that the body can only produce 18grams of glucose from fats, so the rest will be created via gluconeogensys/protein breakdown at the rate of 58% , and this is what we want to avoid (loss of body protein). He says that for the first days up to the 3 week mark we need to be consuming 150g down to 50g ( at the 3 week mark our glucose needs are lowered a lot) of protein so that the body doesn't convert body protein to fill that glucose requirement gap.

My question is, why not consume carbs/glucose at a 100% conversion rate (into glucose) to fill this gap? Why do we need massive amounts of protein at conversion rate (into glucose) of 58% to fill this glucose deficit?

Overthinking it. You need to eat protein to prevent muscle loss. Protein just so happens to be the #1 macro-nutrient for protein sparing.

Also a lot of people find it easier to meet protein requirements on a keto diet. With carbohydrate limitation, you have more calories accessible for fats and proteins: fattier cuts of meats and fish come to mind.